Other service supervisors implement systemd's readiness notification
protocol. For example, s6 implements it with the
s6-notify-fd-from-socket[1] program. libsystemd::daemon::notify
already checks for NOTIFY_SOCKET being set, which should be sufficient
to tell if systemd-style readiness notification is wanted.
Link: https://skarnet.org/software/s6/s6-notify-fd-from-socket.html [1]
